Chromium plating



Patented Oct. 18,1927.

UNITED STATES PATENT OFFICE.

HARBIE C. PIERCE, 01 INDIANAPOLIS, INDIANA, ASSIGNOR TO METALS PROTECTION CORPORATION, OF INDIANAPOLIS, INDIANA, A CORPORATION 01 INDIANA.

cnnomom rha'rme.

No Drawing.

This invention relates to chromium pl ating; and it comprises a method of obtammg coatings-of chromium on other metals, and particularly iron and steel, by electrodepo- 5 sit-ion wherein the article to be plated is temporarily made an anode in a plat ng bath and is then made a cathode and plated; all as more fully hereinafter set forth and as claimed.

In electroplatin the ordinary metals and alloys, such as si ver, gold, copper, nlckel, zinc, cadmium, brass, etc., there are no particular problems in an operative or manipulative way. While the baths and electrical conditions vary, the procedure in platmg the metal is always much the same. The article is properly cleaned accordin to standard plating room methods and 1s t en suspended in the plating solution from rods serving as cathodes, the current usually not being on at the time of suspension, though in some cases the current may be applied while the work is being hung. In a general way, 1t may be said that the surface of any of these metals 5 sufi'ers no change here important in the preliminary o erations before the actlon of the current ma es it a cathode in a plating bath. And in chan 'ng from one metal to another, no unusual p enomena are observed requ rin any variation from this standard proce ure.

Conditions, however, are quite different 1n plating chromium and particularly w1th the most successful type of bath, that using chromic acid, or OrO as an electrolyte; what takes place at the surface of the metal when it first contacts with the bath is imortant. This solution is one of a highly oxidizing nature, wherein it differs from most 40 of the common electroplating baths. And it makes an important difference in the procedure whether the article is suspended from the cathode rod with the current on or off. Surfaces of iron, steel or nickel brought into contact'with a solution of chromic ac1d with the current 05 apparently assume a passive condition; this passivity however not being uniform. Localized areas differ from each other. Where passivity exists deposition of plating metal is resisted. I have found that very high initial plating voltages are required to break down this pass vity and permit deposition of the chromium. And where the shape of the article to be plated is such as to ofi'er local high relief and chromic acid solutions,

Application filed March 5, 1926. Serial No. 92,615.

local low depressions, even with these high initial voltages, there is a tendency at times towards retention of the passive condition" any electroplating bath to produce a thicker coating on portions in relief is considerably accentuated in Working with chromium. Occas onally, it happens that an article remain ng, in the plating bath for a normal platmg period receives absolutely no chromlum 1n depressions and it is very frequently the case that the depressions receive merely a film coating. This phenomenon in chrome plating baths requires constant attention if commerclal chrome plating is to be effected. Flaws and faults resulting from defective platlng are in many cases serious, since the chromlum coating is employed in many cases as a protection a ainst corrosion, erosion and abrasion. Artic esto be protected must be completely and evenly covered with chromlum. The stated trouble occurs toa greater extent where the article is put into the'bath with the current oil, but it occurs to some extent even when the article is placed in the bath with the current on. And there are practical difficulties contingent upon hanging articles in the bath with the current on, this being partly because of the high current densities used. It is a well known fact that while giving good deposits, have but a low throwing power va ue It is the purpose of the present invention to obviate the stated dilficulties and make conditions in chrome plating more nearly analogous to those in ordinary plating; that is, so that the article to be plated can be placed and handled in the bath as readily as 1n ordinary plating operations. To this end, I subject the article to be plated to a certain surface-preparing treatment, preliminary to the actual plating. Prior to the plating process proper, in which the metal is, of course, made a cathode, I electrically treat its surfaces in various ways; in particular making the surface to be plated an anode for a time. The article to be plated should be cleaned according to the ordinary plating practice in a suitable alkaline cleaning solution. Its surface should be free not only of dirt or grease, but of oxide, etc. In the -to be bufied to 00 or after plating, so 'ivisable, as soon as the required t cleaning solution, in order toassure this it is advantageous to make it cathodic, current bein applied for a minute or two with the artic e gassin vigorously The article is then rinsed o in ld water and suspended preferably by an 1I0I1Wl18 in a chromium plating bath of an ordinary composition. in which it ismade an anode, the current being so re lated as to make the article gas slight y.; Here I allow it to remain for a varying time, from a few seconds up to, perhaps, a minute. I have found that when the article is made an anode in one of these solu- 'tions, it apparently reaches a condition of equal passivity or uniformity over its whole surface. Atall events, when'the current is subsequentl reversed; that is when the article is Ina e a cathode in a chromium plating bath,- the deposit which forms is now muchmore even and is thrown into the recessed and shaded portions in a very desirable-xway. f The bath in which the article is made anodic may bethe same'as the bath'in which plating is suhse uently effected, making the articlea catho e, or 1t may be a separate bath used only for this preparatory treatment.

In some cases Ihave found it advantageous to make the-article a cathode .in a chromium platin bath prior to-making itan anode; that 1s, to.throw down on it a little coating which is taken off again in the anode stage. In other words in my preparatory treatment instead of continuously maintaining the current in such direction as to make the article cathodic, it may be intermittently reversed. In so doing, in the final anode stage the treatment should be continued somewhat longer; occasionally as long as three minutes, since it isnecessary to strip the coating made before'in addition to ad-" justing surface conditions.

The chrome plating maybe done with the article cathodic in an of the ordinary ways. Usually, and with t ,e-average bath, there are required about 'ampe'res per square foot, a though the platin range'umay be from 40 to 200 am eres. ere an'article is it is ad: ickness of an electrolytic chrome platin chronium deposit has been a proximately rarily making such a surface an'anode in a chrome plating bath and then makin it a cathode in a chrome plating bath and epositing a layer of chromium thereupon.

' 2. In the electrodeposition of chromium on articles having a surface of iron, steel or nickel, the process which comprises temporarily making such a surface a cathode in a cleaning bath then temporarily an anode'in bath and thereafter making it a cathode in a chrome plating bath and depositing a layer of chromlum thereupon.

3. In the electrodeposition of chromium on articles having a surface of iron, steel or nickel, the process which comprises temporarily making such a surface a cathode in a cleaning bath, then temporarily an anode in an electrolytic chrome plating bath with occasional reversals to make it temporarily a cathode, and thereafter making it a cathode in a chrome plating bath and depositing'a layer of chromium thereupon.

4. A process of plating a metal article with chromium in an electrolyte containing chromic acid,-which consists in directin an electric current first through the artice as an anode andthen through the electrolyte to an electrode contacting with the electrolyte, and then reversing the current flow so that it passes throu h said electrode and then throu h the e ectrolyte and the article to be plate said article thus being made the cathode in the bath.

f In testimony whereof, I have hereunto affixed my signature.

HARRIE C. PIERCE. 

